1. Field of the Invention
The invention relates to new hydrophilic/hydrophobic polyurethanes which are soluble or dispersible in water, suitable as thickeners for aqueous systems and distinguished by having a particularly efficient thickening action, and to their use for the thickening of aqueous systems.
2. Description of the Prior Art
Polyurethane-based thickeners for aqueous systems are described in numerous publications (cf., for example, DE-OS 1,444,243, DE-OS 3,630,319, EP-A 0,031,777, EP-A 0,307,775, EP-A 0,495,373, U.S. Pat. No. 4,079,028, U.S. Pat. No. 4,155,892, U.S. Pat. No. 4,499,233 and U.S. Pat. No. 5,023,309).
These prior art thickeners have in common the simultaneous presence of (i) hydrophilic segments in a quantity of at least 50% by weight, (ii) hydrophobic segments in a quantity of at most 10% by weight and (iii) urethane groups. "Hydrophilic segments" are understood in this connection to be polyether chains having at least 5 alkylene oxide units, in which the alkylene oxide units contain at least 60 mol-% of ethylene oxide units. "Hydrophobic segments" are understood in this connection to be in particular hydrocarbon segments having at least 6 carbon atoms incorporated inside the chain and/or preferably terminally.
The thickeners according to the invention described below also preferably correspond to this definition.
These polyurethane thickeners are suitable as auxiliary substances for adjusting the flow properties of aqueous systems such as automotive and industrial coatings, finishes and paints, printing inks and textile dyes, pigment printing pastes, pharmaceutical and cosmetic compositions, formulations for plant protection and filler dispersions.
Although the known polyurethane thickeners are widely used, they have a thickening action that is too low for many fields of application. As a result they must either be used in comparatively high concentrations or else other measures must be taken to increase the viscosity, such as increasing the concentration of the pigment or solids. However, this may lead to undesirable changes in the properties associated with the application of the paints or other formulations prepared, for example, flow, curing behavior, gloss or hiding power.
A particular problem in the use of polyurethane thickeners is that they must have an effective action not only at low shear rates (which is important for the settling behavior and flow of paints), but also at high rates of shear (high shear range) which occur when formulations are applied, for example, with a brush or a roller or by spraying. For this reason in the prior art as a rule, two basic types of thickeners are used in one formulation for the respective ranges of shear rates, or otherwise auxiliary substances are added, such as solvents, in order to reduce the low-shear viscosity.
In the past many attempts were made to improve the effectiveness of aqueous polyurethane thickeners. Thus, for instance, the thickening action at low shear rates (low shear range) could be improved by increasing the length or the proportion of hydrophobic end groups. However, the effectiveness of such thickeners in the high shear range is inadequate for many fields of application.
On the other hand, improvements were attained in the high shear range, for example, by the incorporation of substituted ethane diols or ethylene diamines having long hydrophobic side chains. However, such thickeners showed inadequate effectiveness in the low shear range.
These thickeners therefore have the feature in common that they contain as hydrophobic groups either terminal alkyl groups and/or branched groups incorporated in the polymer chains by the reaction of hydrophobic short-chain diols or diamines containing lateral groups with, for example, diisocyanate prepolymers.
An object of the present invention is to provide new thickeners based on polyurethanes for aqueous or predominantly aqueous systems which have a well-balanced thickening action both in the low shear range and in the high shear range.
This object may be achieved with the water soluble or water dispersible, hydrophilic/hydrophobic polyurethanes according to the invention described in more detail below. Essential to the invention is the incorporation of selected hydrophilic or hydrophobic segments using a specific mixture of alcohols as co-reactant for the isocyanate component.